I'm considering buying an unlocked G6 (US997) to use on AT&T. I've read that it doesn't support LTE band 30 and may not work as well on AT&T as the carrier version. Is this correct.
It appears that your information is correct. The AT&T model is the only one that has LTE Band 30 enabled.
All is not lost, however. You will probably still have coverage on another LTE band but it may be a more saturated LTE band, and HSPA/HSPA+ should still work fine (albeit not as fast as LTE, of course!).
You won't get insane AWS/LTE performance, but around here (Maine) I rarely even see anything as fast as HSPA can offer coming across any LTE band, and with a 3GB plan it's just that many fewer minutes until I can eat up my monthly allotment. LOL
I'm in Alabama with an unlimited data plan. I think I'll pass on the US997. I like the idea of no carrier bloat but prefer the fastest connection available.